Repetitive electrical impulse noise (REIN) can affect communication systems and can comprise a plurality of impulses, each impulse having a duration, wherein impulses are separated by regular time periods or intervals. Electric power lines carrying AC power are one mechanism that can cause REIN, wherein the REIN impulses typically appear with twice the frequency of the AC power. Depending on the country, typical AC power systems can have, for example, a frequency of 50 Hz or 60 Hz. Such AC power systems would result in REIN with a frequency of 100 Hz or 120 Hz, respectively, which corresponds to a REIN period of 10 ms or 8.33 ms, respectively. Further, such REIN impulses could have various durations, for example, in the range of approximately 50-500 μs or of approximately 1-2 ms. Although electric power lines are one mechanism that could cause REIN, countless other mechanisms could also do so.
Communication systems, including digital subscriber line (DSL) communication systems, can be severely affected by REIN. Specifically, DSL modems using discrete multi-tone (DMT) modulation (e.g., ADSL or VDSL) are affected severely by REIN because a single REIN burst may alter a complete DMT symbol. If REIN alters a complete symbol, a large number of bit-errors will occur. For example, if the length of a DMT symbol is 0.25 ms, then REIN could alter on average one in every 33.3 (for 120 Hz REIN frequency with period 8.33 ms) or one in every 40 (for 100 Hz REIN frequency with period of 10 ms) DMT symbols. This error rate may not meet the quality of service requirements for DSL systems.
FIG. 1 shows one known communication protocol 100 for providing communication service, wherein horizontal lines can indicate communication signals or messages exchanged between modems 102, 104 and over a twisted pair of copper wires 106. The illustrated communication protocol 100 includes several steps, which are further described below, namely: handshake 108, initialization 110, and data transfer 112.
In step 108, the modems can undergo a handshake, which is somewhat analogous to two people shaking hands and introducing themselves when they first meet. The handshake serves to identify the type of modems in communication, and to negotiate and agree on a common set of initial communication parameters (e.g., bit distribution parameters) for the communication service, wherein handshake messages are sent from one modem to the other to facilitate the negotiation and agreement of the common set of communication parameters. In one embodiment, the handshake may be carried out in accordance with ITU-standard G.994.1.
After the handshake, the modems can continue on to an initialization in step 110. The initialization can include a message exchange and the capture of channel parameters (e.g., channel estimation, signal-to-noise ratio estimation, calculation of coefficients of the equalizer and echo canceller). In a typical situation, the signal-to-noise ratio (SNR) for each sub-carrier is measured during system initialization, and the maximum bit capacity of each sub-carrier is determined. Once the transmission capability of the system is thus assessed, the modems can assign more bits (e.g., larger constellation sizes) onto sub-carriers with higher SNR compared to sub-carriers having lower SNR and the sub-carrier relative transmit powers (e.g., gains) are set. Illustrative initializations are described in ITU-T recommendation G.992.1 (ADSL) or ITU recommendation G.993.2 (VDSL2).
After the initialization 110, the modems can transfer data in 112. In one known embodiment, the modems do not protect messages with impulse noise protection (e.g., FEC encoding, interleaving) until data transfer. In other words, although known DSL systems use impulse noise protection in data transfer 112, they do not use impulse noise protection in either the handshake 108 or initialization 110. Thus, known methods for protecting against REIN are not applicable during initialization.
In real-world situations REIN may be present all the time—even during the handshake or initialization stage of a communication protocol. However, the prior art fails to disclose a communication protocol that accounts for the presence of REIN prior to data transfer. Accordingly, the need exists for methods to detect the presence of repetitive electrical impulse noise (REIN) and for methods that limit the effects of repetitive electrical impulse noise.